"Two billion years ago, around the time atmospheric oxygen levels were rising, one cell engulfed another, and instead of becoming lunch, the ingestee became an Earth-changer and, eventually, a vital part of you: mitochondria."
"These microscopic cell inhabitants/engines allowed their host cell to suddenly begin to burn oxygen when digesting their food, an energy source that vastly expanded the amount of energy they could harvest from a given morsel of food. The magic born of this union helped enable nearly all multicellular life on Earth to evolve and get big, complicated, and, in our case, hairy and prone to back problems.
Most multicellular organisms would agree it was a good move. However, there have long been fights among biologists as to whether this aborted lunch was actually the source of mitochondria, and then, when that was all settled, what the identity of the engulfer and engulfee actually were."